In radiotherapy, image data is generated by imaging during treatment planning, and treatment plan data is generated based on the image data. In addition, image data is generated by imaging just before the treatment. Then, the image data obtained just before the treatment is aligned with the image data for the treatment plan to compute a difference of the image data obtained just before the treatment from the image data for the treatment plan, and then repositioning is carried out by shifting the patient by the difference from the imaging position just before the treatment. After the repositioning, radiotherapy is carried out by irradiating a treatment site of the patient.
Examples of conventional arts relating to the embodiments include Japanese Patent Application Publication (Laid-Open: KOKAI) No. 2010-69086.
However, according to the conventional arts, because both images are aligned with each other based on shading (CT values, image density, values of luminance, or the like) of overall data of both the images including sites unrelated to treatment, shift of internal organs in the data of both the images tends not to be taken into account and an area of interest such as a treatment site may not be aligned with high accuracy. In particular, in the case where chest and abdominal organs such as a lung, a liver, and the like are irradiated, because of respiratory movement of an affected part, a normal site other than a treatment site might be irradiated.